Many commercial and industrial processes generate large amounts of waste heat which must be removed for successful operation. The waste heat is often carried in the form of a hot fluid stream. For a number of reasons, it is often undesirable or impermissible for the hot fluid stream to be disposed of, so it must be cooled and reused. One such hot fluid stream is exhaust steam, such as from an electric power generating steam turbine, which is condensed to water which then is reconverted to steam in a boiler to be used again in powering the turbine.
A base or primary cooling system of one type or another is provided for cooling the hot exhaust steam. All such systems rely, ultimately, on heat rejection to the environment, either by direct rejection or indirectly through an intermediate fluid, to air or to water from a river, lake or sea.
A typical cooling system can be illustrated further by reference to a power generating plant. In the production of electric power, heat is first produced by nuclear energy or combustion of a fossil fuel such as oil, gas or coal. The heat produced is then used to convert water into steam. The steam is conducted at high pressure to a turbine which it drives. The turbine is, of course, coupled to a generator which produces electric power. The spent steam from the turbine is condensed by the cooling system and then the water is recycled and reheated to steam again.
An air-cooled system is generally designed and built to provide a cooling capacity or duty adequate for the intended purpose on the hottest day, or ambient temperature, anticipated at the site of the plant involved. This results in an excess cooling capacity for all but a small number of days out of a year. Even on the hottest days, the maximum cooling capacity of the system often is not utilized except during the very warmest part of the day. The cooling system installation, operation and maintenance involve large costs and expenses which cover a system that is not fully employed, regardless of the hot fluid stream to be cooled.
Various dry-type cooling systems have been proposed. In one such system, the refrigerant ammonia is used in a closed loop cycle to absorb heat in the steam condenser of a power generating plant and then reject heat in a cooling tower where air absorbs the heat from the ammonia coolant. Such a system would require a very large cooling tower investment to provide adequate cooling capacity on very hot days or period of days. This would entail a high capital investment. To reduce this cost, it has been proposed in Husain et al U.S. Pat. No. 4,270,358 to reduce the size of the cooling tower and to provide a peak-shaving cooling system using water as the coolant. Cold water would constitute the peak-shaving coolant and once heated the hot water would be stored until it could be cooled when excess cooling tower capacity became available, such as at nighttime when the air temperature would be lower and electric power demand decreases.
At the present time the 330 MW Wyodak Power Plant at Gillette, Wyoming employs a direct condensing cooling system. The exhaust steam is sent directly to a steam condensing system 164 feet wide and 360 feet long utilizing sixty-six 22-foot diameter fans and three 33-foot diameter fans. Each fan is driven by a two-speed motor. Two sets of vacuum steam ducts, each 13 feet 6 inches in diameter, are used to carry the steam from the turbine exhaust to the A-frame forced draft air cooled condensing coils.
The direct condensing method of cooling exhaust steam involves a low initial capital cost; but a plant using it suffers a penalty at high ambient temperatures because the turbine back pressure increases, thereby reducing the total power generating capacity of the plant. A peak shaving cooling system based on water, such as disclosed in Husain et al U.S. Pat. No. 4,270,358, could be utilized in the Wyodak Power Plant. However, a peak shaving system as disclosed in that patent during regeneration cools all of the hot water produced in peak shaving so that the water can be used later in peak shaving. While such a system is highly useful, it can be improved and its efficiency increased.